In recent years, to cope with the use of various grades of gasoline such as regular, unleaded and premium which are assigned an octane rating for an internal combustion engine, it has increasingly become popular to change engine operating conditions or engine control values, for example ignition timing, supercharging pressure, etc., according to the grade of gasoline delivered to the internal combustion engine.
Detection of the octane rating of a gasoline in use can be effected in various manners. One popular manner of identifying the octane rating of gasoline is to detect knockings occurring in the internal combustion engine. For this, it is common to detect a knocking or knockings in different two engine operating zones: a first zone where the internal combustion engine operates under a relatively high engine load and at a speed under a specific engine speed, and a second zone where the engine operates at a speed over the the specific engine speed. The specific engine speed is so determined that the loudness of noise produced in the engine operating at the specific engine speed does not affect the detection of engine knockings.
Engine knockings caused in the second operating zone are more apt to have an adverse effect on the performance of an engine. Engine knockings caused in some engine operating conditions, although inducing no trouble in the engine operating in the same engine operating condition in the first operating zone, can possibly cause, but seldom actually cause, damage to the engine in the second operating zone. An engine operating condition is, on the other hand, possibly changed directly into the second operating zone without passing through the first operating zone. If an engine control value or values for the engine, although a low octane rating of gasoline is practically used in the engine, has or have been set suitably for a high octane rating of gasoline, the engine is more apt to cause knockings, resulting in difficulty in ensuring the performance of engine.
In an attempt at avoiding that difficulty, it is proposed in, for example Japanese Unexamined Patent Publication No. 60(1985)-216,067 entitled "Ignition Timing Control System for Internal Combustion Engine" laid open Oct. 29, 1985, that an engine control value in the second operating zone should be initially set and maintained suitably for a low octane rating gasoline unless the gasoline in use is determined to be of a high octane rating gasoline when the engine operating condition is changed to the first operating zone.
To realize a high efficiency of the use of engine output, it is generally preferable to set the engine control value suitably for a high octane rating of gasoline as long as a high octane rating of gasoline is practically used. However, from the standpoint of the reliability of performance of an engine in the second operating zone, unrestricted change of the engine control value to a value suitable for a high octane rating of gasoline should be avoided.